Small line width tunable laser

Abstract

For small line width tuning of a laser, especially a pulsed dye laser, a reference beam is uncoupled from the laser resonator (10,18) and directed to a beam position photosensor such as an adjacent pair of photo diodes. A control signal for synchronizing the movements of a grating (10) and an etalon (18) of the laser resonator is derived from a local change of the reference beam in order that synchronism may be achieved in the movement of both the etalon and the grating.

I claim: 
     
       1. A tunable laser comprising: at least two wavelength selecting members in a given path of a resonant beam of variable wavelength, said at least two wavelength selecting members being rotatable to specific wavelength-dependent angular orientations with respect to said given path to maximize the intensity of said beam at a chosen wavelength, wherein relative misorientation of one of said wavelength selecting members with respect to the other of said wavelength selecting members causes a divergence of said resonant beam from said given path;   first control means for varying the orientations of said wavelength selecting members to vary the wavelength of said resonant beam over a range of wavelength values to a chosen wavelength;   detector means for providing a control signal indicative of said divergence; and   second control means responsive to said control signal for operating said first control means to reduce said divergence.   
     
     
       2. The laser of claim 1 wherein said detector means includes reference beam means for producing a reference beam derived from said resonant beam and following a reference beam path which varies in direction from a given reference beam path responsively to said divergence, and; optical sensing means disposed in said path of said reference beam and including means responsive to changes in said direction of said reference beam for providing said control signal.   
     
     
       3. The laser of claim 2 wherein said first control means includes first motor drive means for bidirectionally rotatingly driving one of said wavelength selecting members over a range of angular positions and second motor drive for bidirectionally rotatingly driving another of said wavelength selecting members over a range of angular positions, said optical sensing means includes means responsive to changes in the direction of said reference beam in a first direction away from said given reference beam path for producing an overrotation-indicating signal condition indicative of overrotation of said one of said wavelength selecting means with respect to the other of said wavelength selecting means, and means responsive to changes in the direction of said reference beam away from said given reference beam path in a second direction different from said first direction for producing an underrotation-indicating signal condition indicative of underrotation of said one of said wavelength selecting members with respect to said other of said wavelength selecting members, and said second control means includes means responsive to said overrotation-indicating and underrotation-indicating signal conditions for controlling said first motor drive means to move said reference beam towards said given reference beam path. 
     
     
       4. The laser of claim 3 wherein said optical sensing means includes first and second photosensing systems disposed to be equally excited when said reference beam follows said given reference beam path and to be unequally excited so as to indicate in which direction said reference beam has moved away from said given reference beam path, said photosensing systems including means for producing respectively said overrotation-indicating and underrotation-indicating signal conditions responsively to overexcitation and underexcitation of said first photosensing system with respect to said second photosensing system. 
     
     
       5. The laser of claim 4 wherein one of said wavelength selecting members is a diffraction grating and the other of said wavelength selecting members is an etalon. 
     
     
       6. The laser of claim 2 including a beam splitter disposed in said resonant beam path and configured to split out said reference beam from said resonant beam. 
     
     
       7. A method for controlling a tunable laser having at least two wavelength selecting members in a given path of a resonant beam of variable wavelength, said at least two wavelength selecting members being rotatable to specific wavelength-dependent angular orientations with respect to said given resonant beam path to maximize the intensity of said beam at a chosen wavelength, wherein relative misorientation of one of said wavelength selecting members with respect to the other of said wavelength selecting members causes a divergence of said resonant beam from said given resonant beam path comprising the steps of: sensing said divergence of said resonant beam; and   adjusting the orientation of at least one of said wavelength selecting means responsively to said sensing to restore said resonant beam to said given resonant beam path.   
     
     
       8. The method of claim 7 including the steps of deriving a reference beam from said resonant beam so as to deviate from a given reference beam path responsively to said divergence of said resonant beam, and adjusting the orientation of said at least one wavelength selecting members according to the deviation of said reference beam to restore said reference beam to said given reference beam path. 
     
     
       9. The method of claim 8 wherein said laser includes a beam splitter disposed in said resonant beam path and configured to split out and thus derive said reference beam from said resonant beam.